1. Field of the Invention
The present invention relates to a polymerizable compound and a polymer. In particular, the present invention relates to a novel polymerizable compound that is very useful industrially, and a polymer that comprises a copolymer unit derived from the polymerizable compound and is used as a pigment dispersant to be used for color filters, inkjet ink, etc. The present invention also relates to an ink composition suitably used for inkjet recording, a printed article using the ink composition, and an inkjet recording method using the ink composition. In particular, the present invention relates to: an ink composition suitable for inkjet recording, the ink composition being superior in the dispersibility of a colorant so that it has excellent coloring property, being cured by irradiation with an active radiation ray without generating volatile components and enabling the formation of a high quality image; a printed article obtained using the ink composition, and an inkjet recording method using the ink composition.
2. Description of the Related Art
As an image recording method for forming an image on a recording medium such as paper based on image data signals, there are an electrophotographic system, sublimation-type and melt-type thermal transfer systems, an inkjet system, etc. In particular, the inkjet system is applicable to an inexpensive apparatus, and performs direct image formation on a recording medium by ejecting ink only to image areas where ink deposition is necessary; therefore, the inkjet system uses ink effectively, thus reducing the running cost. Further, the inkjet system generates less noise and is a superior image recording system.
The inkjet system enables printing on recording media that do not absorb water, such as plastic sheets and metallic plates, as well as on paper. In the inkjet system, it is an important target to achieve higher printing speed and higher printing quality, and the time that liquid droplets take to dry and cure after printing has significant influence on the sharpness of the image. In one inkjet system, an inkjet recording ink that can be cured by irradiation with an active radiation ray is used. According to this system, sharp images can be formed by curing ink droplets by irradiation with an active radiation ray immediately after printing.
In order to form highly accurate images with excellent coloring properties, the curable ink composition in such an inkjet recording ink that can be cured is required to have high dispersibility of a pigment, and stability over time. Reduction in the pigment size is necessary for imparting clear tone and high tinting strength to the ink composition. In particular, ejected droplets of the inkjet ink exert a significant influence on the sharpness of images; therefore, the ejection quantity of the ink droplets needs to be small, and the size of the pigment particles contained in the ink has to be smaller than the thickness of the film formed by curing of the ink. However, when the size of the pigment particles is reduced so as to achieve higher tinting strength, the pigment particles are hard to disperse, and pigment aggregates are easily formed. Another problem is created in that the viscosity of the ink composition is increased by the addition of a dispersant. The formation of the pigment aggregates and the increase in the viscosity of the ink composition both adversely affect ink ejection property, and such an ink composition is not preferred.
When an ink composition is used in inkjet recording, the ink composition is contained in a cartridge. The ink composition in the cartridge is heated at ejection and cooled at a non-ejection time and at storage; in this way, the ink composition undergoes repeated temperature changes (heating-cooling). This temperature changes also adversely affect the pigment dispersibility, and the pigment dispersibility is deteriorated with time, causing problems that pigment aggregates are easily formed, or the increase in the viscosity of the ink composition easily occurs.
Accordingly, there is a demand for an ink composition having sufficient fluidity and excellent stability of the pigment dispersion over time, in which fine colorant particles are dispersed stably. Various proposals have been made concerning dispersants for achieving stable pigment dispersion liquids.
In order to improve compatibility with the pigment, an ink composition using a pigment derivative as a dispersant (see, for example, Japanese Patent Application Laid-pen (JP-A) Nos. 2003-119414 and 2004-18656); an ink composition that uses a polymer having a basic group as a dispersant for a specific pigment such as a phthalocyanine-based or quinacridone-based pigment (see, for example, JP-A No. 2003-321628); and an ink composition which is free of organic solvent and contains a dispersant such as poly(ethyleneimine)-poly(12-hydroxystearic acid) graft polymer and a specific monomer that dissolves the dispersant (see, for example, JP-A No. 2004-131589) have been proposed.
In these ink compositions, the dispersion stability of pigments is actually improved owing to the function of the dispersant. However, the size of the pigment used in the ink composition is not sufficiently small, and there is a room for improvement of dispersibility of finer pigment particles. Further, the ink composition also has a problem in that the dispersion stability upon long-term storage or upon repeated temperature changes is still insufficient.